There is a considerable interest in epidemiology in evaluating the relationship of physical activity to chronic disease. Little is known, however, about the reliability and validity of instruments designed to measure physical activity in epidemiologic research. The current proposal examines one measure, the Large-Scale Integrated activity monitor to determine its reliability, validity and relationship to other physical activity tools. Four experiments are proposed to evaluate the Large-Scale Integrated (LSI) activity monitor as an objective measure of physical activity. Experiment 1 evaluates the instrument in relationship to various recreational activities, as well as to other commercially available motion sensors. Experiment 2 examines the relationship between the LSI activity measurement, commercially available monitors and the Paffenbarger and Taylor surveys as well as determining the test-retest reliability of the instruments in a population of normal middle-aged men. Experiment 3 evaluates the precision of the instruments for detecting individual differences in activity in a population of individuals with an extremely low level of activity (people with spinal cord injuries). Additionally the interest is the relationship of the activity measures in extremely low activity populations. Experiment 4 examines the validity of the LSI activity monitors as well as the Paffenbarger survey for quantifying physical activity in an extremely active population (postal carriers). In this population it will be possible to directly relate the activity measures to distance walked. The research therefore will further test the reliability and validity of the LSI for monitoring activity across the total spectrum of activity from extremely low levels of activity through typical daily activity to extremely high levels of activity in postal carriers.